diff --git a/packages/editor-ext/src/lib/html-embed/html-embed-codec.spec.ts b/packages/editor-ext/src/lib/html-embed/html-embed-codec.spec.ts
index 917f1d51..f50bec0f 100644
--- a/packages/editor-ext/src/lib/html-embed/html-embed-codec.spec.ts
+++ b/packages/editor-ext/src/lib/html-embed/html-embed-codec.spec.ts
@@ -2,6 +2,8 @@ import { afterEach, describe, expect, it } from "vitest";
 import {
   encodeHtmlEmbedSource,
   decodeHtmlEmbedSource,
+  parseHtmlEmbedHeight,
+  renderHtmlEmbedHeight,
 } from "./html-embed";
 
 // Unit coverage for the base64 codec used by the htmlEmbed node's
@@ -118,6 +120,45 @@ describe("html-embed codec — encode failure fallback", () => {
   });
 });
 
+describe("html-embed height — parseHtmlEmbedHeight (data-height -> px | null)", () => {
+  it('parses a numeric string ("300" -> 300)', () => {
+    expect(parseHtmlEmbedHeight("300")).toBe(300);
+  });
+
+  it("parses an absent value (null -> null = auto-resize)", () => {
+    expect(parseHtmlEmbedHeight(null)).toBeNull();
+    expect(parseHtmlEmbedHeight("")).toBeNull();
+  });
+
+  it('rejects a non-numeric value ("abc" -> null) — pins the NaN guard (BUG-2)', () => {
+    // Without Number.isFinite this would be NaN (typeof "number"), disabling
+    // auto-resize and yielding an unclamped iframe height downstream.
+    expect(parseHtmlEmbedHeight("abc")).toBeNull();
+  });
+
+  it('parses a trailing-unit value ("120px" -> 120) via parseInt', () => {
+    expect(parseHtmlEmbedHeight("120px")).toBe(120);
+  });
+});
+
+describe("html-embed height — renderHtmlEmbedHeight (px -> data-height | {})", () => {
+  it("renders a fixed height (120 -> { data-height: '120' })", () => {
+    expect(renderHtmlEmbedHeight(120)).toEqual({ "data-height": "120" });
+  });
+
+  it("renders auto-resize as no attribute (null -> {})", () => {
+    expect(renderHtmlEmbedHeight(null)).toEqual({});
+  });
+
+  it("renders 0 as no attribute (0 is auto -> {})", () => {
+    expect(renderHtmlEmbedHeight(0)).toEqual({});
+  });
+
+  it("renders undefined as no attribute (absent -> {})", () => {
+    expect(renderHtmlEmbedHeight(undefined)).toEqual({});
+  });
+});
+
 describe("html-embed codec — decode of malformed input (browser branch)", () => {
   it("returns '' for input atob rejects (catch branch)", () => {
     // atob throws on characters outside the base64 alphabet; the codec catches
diff --git a/packages/editor-ext/src/lib/html-embed/html-embed.ts b/packages/editor-ext/src/lib/html-embed/html-embed.ts
index baa396e1..c0bbfe81 100644
--- a/packages/editor-ext/src/lib/html-embed/html-embed.ts
+++ b/packages/editor-ext/src/lib/html-embed/html-embed.ts
@@ -69,6 +69,30 @@ export function decodeHtmlEmbedSource(encoded: string): string {
   }
 }
 
+/**
+ * Parse the `data-height` attribute value into a fixed iframe height in px.
+ *
+ * Returns null (auto-resize) when the value is absent, empty, or non-numeric.
+ * A non-numeric `data-height` (e.g. a crafted/corrupted import) must NOT become
+ * NaN: NaN is typeof "number" and would disable auto-resize and yield an
+ * unclamped iframe height downstream. The Number.isFinite guard pins that fix.
+ */
+export function parseHtmlEmbedHeight(value: string | null): number | null {
+  if (!value) return null;
+  const n = parseInt(value, 10);
+  return Number.isFinite(n) ? n : null;
+}
+
+/**
+ * Render a fixed height back to a `data-height` attribute. A null/0/absent
+ * height means auto-resize, so no attribute is emitted.
+ */
+export function renderHtmlEmbedHeight(
+  height: number | null | undefined,
+): { "data-height": string } | Record<string, never> {
+  return height ? { "data-height": String(height) } : {};
+}
+
 export const HtmlEmbed = Node.create<HtmlEmbedOptions>({
   name: "htmlEmbed",
   inline: false,
@@ -103,17 +127,9 @@ export const HtmlEmbed = Node.create<HtmlEmbedOptions>({
       // Fixed iframe height in px. null/absent => auto-resize on the client.
       height: {
         default: null,
-        parseHTML: (el) => {
-          const v = el.getAttribute("data-height");
-          if (!v) return null;
-          const n = parseInt(v, 10);
-          // A non-numeric data-height (e.g. crafted/corrupted import) must not
-          // become NaN: NaN is typeof "number" and would disable auto-resize and
-          // yield an unclamped iframe height downstream. Treat it as auto (null).
-          return Number.isFinite(n) ? n : null;
-        },
+        parseHTML: (el) => parseHtmlEmbedHeight(el.getAttribute("data-height")),
         renderHTML: (attrs: HtmlEmbedAttributes) =>
-          attrs.height ? { "data-height": String(attrs.height) } : {},
+          renderHtmlEmbedHeight(attrs.height),
       },
     };
   },